Thresholds for interference resistance of pacemakers:
The fixing of these thresholds comprises static magnetic fields and also elctromagnetic fields.
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Low frequency fields
The interference threshold of pacemakers is 6000V/m at 50Hz.
Modulated magnetic fields
The interference threshold with modulated magnetic fields in a lower frequency range <10kHz is 80A/m which corresponds to 100uT. The threshold for higher frequencies >10kHz is 1A/m.
High frequency fields
The threshold for higher frequencies will only be given in V/m because there is a fixed relationship between electric and magnetic fields at those frequencies (> 150kHz). Pacemakers resist to a field of 100V/m at their inputs.
Ron, I think you risk nothing with a normal Theremin operating at low voltage. Even if you would be in front of a vacuum tube theremin with an anode voltage of 75V with the tank circuit in the anode circuit at full amplitude (which is never the case!) you would not reach the 100V/m.
So don't worry and enjoy your theremin.